Please Help - WMP won't play my tagged media...

Hi-

I apologize beforehand if I am missing something simple here, but my entire music library is pretty much useless right now, so any help would be appreciated.
I dowloaded the program because I wanted to add "album art" for songs on my iPod. (BTW - I have Windows Vista.) The program had renamed the songs in my library (with a "-" in front, usually) but now I can't play any of these songs with Windows media player. They are all WMA format now, but I can usually play this format without a problem.
Worse, none of my audio conversion programs will recognize my songs now either. The error the WMP gives me is : "Windows Media Player cannot play the file. The Player might not support the file type or might not support the codec that was used to compress the file."
How can I convert my music back to a format that can be recognized again?

Lars